A systematic multi-isotope approach to unravel methane origin in groundwater: Example of an aquifer above a gas field in southern New Brunswick (Canada)

Bordeleau, Geneviève; Rivard, Christine; Lavoie, Denis; Lefebvre, René

doi:10.1016/j.apgeochem.2021.105077

Cited by 8 publications

(2 citation statements)

References 63 publications

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“…Increasing concerns associated with the expansion of unconventional O&G development have prompted research into long‐term monitoring of unintended migration on natural gases from deeper zones into shallow groundwater (Jackson et al 2013 ; Humez et al 2016 ; Cahill et al 2017 ; Bordeleau et al 2021 ; Li et al 2021 ), which can potentially have negative groundwater quality impacts and increase the risk of asphyxiation and explosion (Kelly et al 1985 ; Jackson et al 2013 ; Vengosh et al 2014 ; Hammond 2016 ). The expansion of carbon capture, utilization, and storage studies (CCUS) has also increased the need for long‐term monitoring of fugitive gases in the subsurface (Boot‐Handford et al 2014 ; Lindeberg et al 2017 ).…”

Section: Introductionmentioning

confidence: 99%

In‐Well Degassing of Monitoring Wells Completed in Gas‐Charged Aquifers

Morais

Ryan

2022

Groundwater

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Total dissolved gas pressure (PTDG) measurements are useful to measure accurate in situ dissolved gas concentrations in groundwater, but challenged by in‐well degassing. Although in‐well degassing has been widely observed, its cause(s) are not clear. We investigated the mechanism(s) by which gas‐charged groundwater in a recently pumped well becomes degassed. Vertical PTDG and dissolved gas concentration profiles were monitored in the standing water column (SWC) of a groundwater well screened in a gas‐charged aquifer for 7 days before and 15 days after pumping. Prior to pumping, PTDG values remained relatively constant and below calculated bubbling pressure (PBUB) at all depths. In contrast, significant increases in PTDG were observed at all depths after pumping was initiated, as fresh groundwater with elevated in situ PTDG values was pumped through the well screen. After pumping ceased, PTDG values decreased to below PBUB at all depths over the 15‐day post‐pumping period, indicating well degassing was active over this time frame. Vertical profiles of estimated dissolved gas concentrations before and after pumping provided insight into the mechanism(s) by which in‐well degassing occurred in the SWC. During both monitoring periods, downward mixing of dominant atmospheric and/or tracer gases, and upwards mixing of dominant groundwater gases were observed in the SWC. The key mechanisms responsible for in‐well degassing were (i) bubble exsolution when PTDG exceeded PBUB as gas‐charged well water moves upwards in the SWC during recovery (i.e., hydraulic gradient driven convection), (ii) microadvection caused by the upward migration of bubbles under buoyancy, and (iii) long‐term, thermally driven vertical convection.

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Section: Introductionmentioning

confidence: 99%

In‐Well Degassing of Monitoring Wells Completed in Gas‐Charged Aquifers

Morais

Ryan

2022

Groundwater

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“…(1976), Schoell (1983), and Whiticar (1999) and updated by Milkov and Etiope (2018). However, these gases are not conservative tracers, with both the molecular and isotopic composition being subject to change as a result of microbial action, mixing between sources, migration (Bordeleau et al., 2021; Schloemer et al., 2018; Whiticar, 1999) and sorption processes (Chalmers & Bustin, 2008; Li et al., 2017; Schloemer & Krooss, 2004). It is therefore necessary that efforts to assign dissolved C 1 to either a biogenic or thermogenic source be informed by a complete understanding of the isotopic and molecular compositional fractionations that may be imparted by microbial, mixing, migration, and sorption processes.…”

Section: Introductionmentioning

confidence: 99%

Reactive Transport Modeling of Natural Gas Molecular and Isotopic Evolution During Diffusive Transport in the Subsurface

Loomer

MacQuarrie

2021

Water Resources Research

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Reactive transport modeling was employed to investigate the relative importance of fractionations associated with gas solubility, sorption and diffusive transport on dissolved methane, ethane and propane concentrations and the isotopic composition of carbon in methane (δ13C1) in groundwater. Temperature, pressure and salinity dependencies for the hydrocarbon gases were incorporated. Gas molecular ratios, C1/(C2 + C3), increased with diffusive transport, transitioning from thermogenic values to values typically indicative of biogenic gas sources, >1,000, at the leading edge of the diffusive front. Diffusive isotopic fractionation had a large effect on δ13C1 values, with fractionations ranging from −36‰ to −107‰, the difference being a function of the diffusive fractionation factor (αD0). Larger fractionations resulted from αD0 determined at relatively low pressures, 5–50 atm, and temperatures, 20°C–25°C. Less fractionation occurred with αD0 measured at higher pressures and temperature, 30–89 atm and 90°C. The extremely depleted δ13C1 values indicated from the modeling, less than −110‰, have not been observed in shallow groundwater, suggesting that diffusive fractionation of δ13C1 is offset by other processes such as microbial oxidation. 2k factorial analysis was used to assess the model sensitivity to specific parameters: estimations of hydrocarbon travel distance are most sensitive to porosity and tortuosity, while the molecular ratio was most sensitive to the free‐water diffusion coefficient, and the isotopic fractionation was most sensitive to αD0. The magnitude of diffusive fractionation on the molecular and isotopic composition of transported hydrocarbon gas may be similar to fractionations from microbial oxidation and mixing between different sources.

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